Various apparatus for quality reproduction of images on plain paper, such as the printing press and electrostatic copier, have been known for hundreds of years. Such apparatus typically operate by exposing selected areas of the paper to ink or toner and preventing exposure of other areas. They work quite well for sharp, high contrast images such as text and line art graphics.
Quality reproduction of images consisting of continuous tones, such as photographs, requires an additional so-called half tone screening step. Half tone screening involves photographing the image through a screen to break it into a grid of small screened dots of varying size. The screen is comprised of many identical elemental apertures, or screen cells--the area of a screen cell thus determines the size of resulting screened dots. The result is that although the screened dots themselves are of solid, uniform color, the variation in size is averaged by the eye to give an appearance of gray tones.
Screening is also used when printing color images to enable separate printing of the primary color components. A separate half tone plate is first made for each of the primary colors. The primary color components are then printed over one another by means of the plates, which serve to selectively control where inks of different colors are applied to the paper. A common problem in color image reproduction is avoiding Moire patterns, which result from interference between the periodic dot patterns printed by the respective screens. Moire patterns may be eliminated by careful control of the relative angular orientation for screens of different colors. Moire patterns may also be avoided by using different screen frequencies (or pitches) for each of the primary colors. The ability to use various screen angles and screen pitches can also be used to improve the quality of monochrome images.
With proliferation in the use of computers in the publishing industry in the last two decades, processes for electronic generation of half tone screened images are also widely used. Generally speaking, such techniques proceed by first electro-optically scanning the image line-by-line and digitizing the resulting electrical signals, to produce a computer-readable representation of the source image. The source image representation consists of a matrix, or array, of digital data values, called pixel values, which correspond to the light intensity of a particular elemental area, or pixel, of the source image. The memory location in which a pixel value is located corresponds to the spatial position of its corresponding pixel in the source image. In the case of a monochrome source image, the source image pixel values may assume a number of different gray values or levels.
Although in an electronic screening system as described herein, no light actually passes from the source image through a screen, an analogy can be made to physical systems whereby the relative size of the screened dot corresponds to the intensity of the light from the source image.
Thus, a mathematical representation for the screen is also developed. This has previously been accomplished by using a second array of optical density values which specify the light transmissive properties of a single physical screen cell. The source image pixel values are then compared to the screen of cell array to generate an indication of how large each screened dot of the printed image should be made.
However, to reproduce the effect of various screen angles and pitches, this comparison process must also include a number of trigonometric calculations to translate and rotate the screen cell array representation with respect to the image. Unfortunately these calculations are fairly computeintensive. As such, prior approaches typically require special hardware to generate the transformations with the required accuracy resolution, at the time the image is rendered on an output device.
What is needed is a way to electronically produce screened half tone images in real time while avoiding the need for specialized hardware to perform these trigonometric calculations.
Another problem typically occurs because of the nature of most present day output devices. Film recorders, raster-scan displays and laser printers are device-dot addressable; that is, each elemental displayable or printable unit, or device dot must be specified.
In the case of a monochrome display, the device dot is rendered by turning it either on or off, and in the case of a laser printer, the device dot is rendered by either coloring the paper black or leaving it white. Thus, in some prior art devices, the comparison of source image pixel values and density values can be partially completed beforehand. That is, given a particular screen cell size, it is possible to determine a screen cell array for a particular source image pixel value, where the elements of the screen cell array specify how a number of adjacent device dots are to be rendered in order to produce a screened dot of the correct size. A screen cell array can thus be precalculated for each of the possible values which a source image pixel may assume: at the time of rendering the source image, the correct screen cell array is then simply used, as specified by the source image pixel value.
However, a given screen cell array must still be translated and rotated, and thus the axes of the device dots will not be necessarily aligned any longer. Thus, the screen cell array often not perfectly aligned with the grid of device dots, any resulting fractional bias of the device dots is often readily visible to the eye in the resulting half tone image and thus additional calculations are necessary to compensate for this.
Furthermore, consider also that as the gray levels across an area of the source image change, the resulting change in screened dot shape should occur in a controlled fashion. For example, in some instances, elliptically shaped dots are preferred to ensure that the dots merge uniformly as the gray level increases above a mid tone value.
A number of difficulties thus exist with electronic production and application of half tone screens when using present day device dot addressable output devices.